At present, various radio access technologies exist in a field of mobile communication and services are provided.
For example, concerning a cellular phone, W-CDMA and CDMA2000 called the third generation are widely used and further, the 3.5 generation such as HSDPA and 1xEV-DO capable of a higher-speed data transfer is also provided. In the description above, W-CDMA is an abbreviation of Wideband Code Division Multiple Access. HSDPA is an abbreviation of High Speed Downlink Packet Access. EV-DO is an abbreviation of EVolution Data Only. Also, within a couple of years, as the 3.9 generation mobile communications system, arrival of LTE (Long Term Evolution) which is with higher speed and has less delay is also expected.
On the other hand, in wireless LAN (Local Area Network), there exist WiFi and WiMAX and so on whose standard is established by IEEE 802 committee. In the description above, IEEE is an abbreviation of Institute of Electrical and Electronic Engineers. WiFi is an abbreviation of Wireless Fidelity. WiMAX is an abbreviation of Worldwide Interoperability for Microwave Access.
These radio access technologies are different in a transmission power, a frequency band and a modulation method and have characteristics respectively in a viewpoint of data transfer rate and a service area. Although its data transfer rate is inferior compared to a wireless LAN, a cellular phone can enjoy stable communication quality covering a wide range. Therefore, it can also cope with a high-speed movement. On the other hand, although its data transfer rate is large, WiFi is basically used within a limited area such as an office, home and a restaurant. Also, WiMAX is expected as a method which includes characteristics of a cellular phone and WiFi.
On the other hand, although there exists a multimode terminal which is operable in part in both of W-CDMA and WiFi, a terminal which a user carries basically supports only one kind of radio access technology for a single terminal. Therefore, even if a network with a radio access technology which a terminal can connect becomes crowded and its communication quality deteriorates, the user had no choice but to keep using it just as it is. Also, outside of a service area of a radio access technology which a terminal can connect, even if other radio access technology is served, the user could not use it. Moreover, when a plurality of radio access technologies were desired to be properly used, the user needed to hold a plurality of terminals.
In an environment where such a plurality of radio access technologies is provided, in order to realize a communication environment with high flexibility, research and development of a software radio technology are advanced. According to this technology, it makes various radio access technologies of which a transmission power, a frequency band and a modulation method are different correspond by rewriting control software of one wireless apparatus.
In order to realize a software radio technology, a terminal needs to recognize a usable radio access network (hereinafter, simply referred to as a network). As one of methods for realizing this, there exists sensing. This is what a terminal itself measures a radio wave status and recognizes a network used in a surrounding. However, such sensing needs to be carried out by a unit of channels of each radio access technology. Further, depending on a radio access technology, because frequency used is different by a country and an area, a range of a sensing target becomes enormous. Because of this, in addition to time for sensing becoming longer and delay time until a network is connected becoming larger, rapid battery consumption in a terminal takes place.
In order to solve such a problem, for example, in non-patent document 1, a technique is disclosed which notifies an available network using common pilot signal which is not dependent on a radio access technology. Such a pilot signal is called CPC (Cognitive Pilot Channel or Common Pilot Channel).
FIG. 17 is a block diagram illustrating an example of a radio access system 1 which embodies an available network notification technique indicated in the non-patent document 1. This radio access system 1 includes a plurality of networks 3-1 to 3-5 (in case of FIG. 17, for example, LTE, W-CDMA and WiFi). Also, this radio access system 1 includes a CPC transmitting station 2 to broadcast (broadcast) CPC. The CPC transmitting station 2 will broadcast CPC to a terminal 4-1 and a terminal 4-2 in CPC coverage 5. Here, a frequency band and a modulation method specified in advance are used for CPC. By first receiving CPC from the CPC transmitting station 2 after a power supply starts, the terminal 4-1 and the terminal 4-2 can know information of a network available for its own.
FIG. 18 is a figure describing characteristics of CPC broadcasted by the CPC transmitting station 2. As information of a network, CPC includes an operator (Operator), a radio access technology (Radio Access Technology: RAT) and a frequency (Frequency). As shown in FIG. 18, this information is provided for each area divided into a mesh (in case of an urban area, for example, 10 meters square). After starting the power supply, the terminal 4-1 and the terminal 4-2 grasps their current positions using GPS (Global Positioning System) and so on. After that, CPC information is received, and information of an available network in a mesh corresponding to the current position of its own is acquired. Further, a network to be connected (operator, radio access technology and frequency) is selected from among the networks given, and connection is made with this. By using a technology disclosed by the non-patent document 1, compared with a case where sensing is performed for various frequency bands, a delay time until connection is made with a network can be made short and also battery consumption in the terminal 4-1 and the terminal 4-2 can be reduced greatly.
Also, in non-patent document 2, a specific technique is disclosed which, in an environment using CPC, selects one from among a plurality of candidates of networks. According to this technique, information about the quality and the capacity of each network (delay, jitter, bandwidth, transmission rate, error rate and so on) is included in CPC information. Such an element is digitized and an objective function is built; and a network whose objective function becomes largest according to an application will be selected. As a result, it becomes possible not only to improve system performance but also to select a network with higher user satisfaction.
In the environment described above where a plurality of networks is intermingled, a form in which a terminal selects an arbitrary network and connects is positioned as one of so-called cognitive radio technology. On the other hand, as a different form of a cognitive radio, a form is studied where a certain radio access network uses a frequency band which is assigned to other system as a secondary use in case the frequency band is not used. It is also studied to apply CPC as a technique for notifying which frequency band is in a free state to a system which performs such a secondary use (base station and terminal). In this case, as described above, information of a network operating in the area concerned is transmitted by CPC. It is possible for a system which performs a secondary use to judge, among frequency bands available for a secondary use, a frequency band which is not indicated in CPC as available for a secondary use, and to select one frequency band from among them. Or, a frequency band which a system which performs a secondary use can use may be transmitted by CPC. In this case, a system side which performs a secondary use selects one frequency band from among frequency bands transmitted by CPC.
Also, as a related technology, patent document 1 describes so-called cooperative sensing. In patent document 1, a technology is disclosed which: in order to raise detection accuracy of a free frequency band, judges whether a frequency band of a usage target is free or not within a whole area where a wireless base station and a radio terminal which perform sensing exist. Specifically, in patent document 1, it is disclosed to exchange information used for judgment of free frequency bands among a plurality of wireless base stations which exist in a predetermined judgment area (for example, within a circle of radius of 5 km with a center at a predetermined wireless base station). Information used for judgment of free frequency band is, for example, a sensing result or a sensing judgment result.